Solved! I fixed the oauth problem finally. Here's what happened:
After installing the 1.8.3 version which was supposed to fix the problem, the error was not corrected. Here is what I then tried, which finally fixed the problem:
1. Install the 1.8.3 fix
2. Go back into the Tweet This-->Automatic Tweeting
3. Click the link that says "click here" to get back into https://dev.twitter.com/apps/new
4. Find the application and delete it (write down the settings)
5. Re-create the application
6. Get the new token keys and fill them in on the Tweet This settings. Be sure to choose the Access Type of read & write
I hope this is a solution for others as well.